WebKit Layout Tests

Layout tests are used to check the correctness of the renderer. It involves loading pages in a test renderer (test_shell.exe) and comparing the rendered output tree against an expected output file. Additionally, there are "pixel tests" which compare an image of the rendered page against an expected file.

If on Windows XP, install the necessary fonts (Windows Vista has all the necessary fonts installed by default). You can do this from Control Panel -> Regional and Language Options -> Languages tab -> check both boxes in the Supplemental languages support section.

Make sure the layout tests are checked out in your tree. They should be located in /src/third_party/WebKit/LayoutTests. If that directory is empty, check your .gclient file, comment out lines that say "src/third_party/WebKit/LayoutTests": None, and re-run gclient sync.

If on Windows, run /src/third_party/cygwin/setup_mount.bat to patch the registry so that the cygwin utilities used by the regression know where to find the root of their directory.

If on Mac:

You probably want to strip the TestShell binary before starting the tests. If you don't, you'll have 5-10 running concurrently, all stuck being examined by the OS crash reporter. This will drag your poor machine to its knees, and probably cause other failures like timeouts where they normally don't occur.

If you don't want to strip TestShell (e.g. you want to run it under gdb), you can use the --gdb flag to TestShell so it works better.

Running the Tests

To run the tests, run

$ ./src/webkit/tools/layout_tests/run_webkit_tests.sh.

Example: To run the layout tests with a debug build of test_shell, but only test the svg tests and run pixel tests, you would run run_webkit_tests.sh --debug LayoutTests/svg.

Tests marked as SKIP in webkit/tools/layout_tests/test_expectations.txt won't be run at all, generally because they cause some intractable tool error. To force one of them to be run, either rename that file or specify the skipped test as the only one on the command line (see below).

To run only some of the tests, specify their directories or filenames as arguments to run_webkit_tests.sh relative to the layout test directory (/trunk/third_party/WebKit). For example, to run the fast form tests, use

$ ./run_webkit_tests.sh LayoutTests/fast/forms

Or you could use

$ ./run_webkit_tests.sh LayoutTests/fast/fo\*

as a shorthand.

As a final quick-but-less-robust alternative, you can also just use the test_shell executable to run specific tests by using (for Windows)

but this requires a manual diff against expected results, because the shell doesn't do it for you.

Linux Note: We try to match the windows render tree output exactly by matching font metrics and widget metrics. If there's a difference in the render tree output, we should see if we can avoid rebaselining by improving our font metrics. For additional information on Linux Layout Tests, please see http://code.google.com/p/chromium/wiki/LayoutTestsLinux.

Mac Note: While the tests are running, a bunch of Appearance settings are overridden for you so the right type of scroll bars, colors, etc. are used. Your main display's "Color Profile" is also changed to make sure color correction by ColorSync matches what is expected in the pixel tests. The change is noticeable, how much depends on the normal level of correction for your display. The tests do their best to restore your setting when done, but if you're left in the wrong state, you can manually reset it by going to System Preferences -> Displays -> Color and selecting the "right" value.

Test Harness Options

This script has a lot of command line flags. You can pass --help to the script to see a full list of options. A few of the most useful options are below:

--debug

Run the debug build of the test shell (default is release; this doesn't matter on linux, on linux the last test_shell you built will be used)

Write all generated results directly into the given directory, overwriting what's there

--startup-dialog

Bring up a modal dialog before running the test (useful for attaching a debugger)

Success and Failure

A test succeeds when its output matches the pre-defined expected results. If any tests fail, the test script will place the actual generated results, along with a diff of the actual and expected results, insrc/webkit/{Debug/Release}/layout_test_results/. Look in there for the expected outcome, the actual outcome, as well as a contextual diff between the two. The full expected results are in the src/webkit/data/layout_tests or alongside their respective tests.

A test that runs but produces the wrong output is marked as "failed", one that causes the test shell to crash is marked "** CRASHED **", and one that takes longer than 10 seconds to complete is aborted and marked "** TIMED OUT **". A row of dots in the script's output indicates one or more tests that passed.

There are some failures that are platform differences between our webkit implementation and Apple's implementation. That is, we pass the test, but it shows up as a failure because the text diff or the pixel diff are slightly difference. For example, we render our form controls to look like platform-native widgets while Apple uses Aqua widgets. In these cases, we want to rebaseline the test by generating new expected results. See below for instructions on how to rebaseline.

Known differences with our implementation that we should just rebaseline:

Font sizes - We try to match other Windows browser for font sizes. This sometimes causes text to wrap between different words. This is also ok.

Form controls - On Windows, we use a generic set of widgets rather than aqua styled controls (or the platform's native widgets); on Linux, we use the native widgets. Both of these are different in size and appearance from WebKit's widgets.. Also, becaues of this, our select controls and buttons have a border that show up in the render tree output. This is also ok.

Scrollbars - We use different scrollbar widgets (just like the form controls). We match the size of Apple's scrollbars so the render tree diff should match. However, pixel diffs will fail so it's ok to rebaseline the images.

V8 exceptions - In general, we try to match javascript exceptions exactly with JSC. However, sometimes this is hard or undesirable. If a test is failing only because exception text differs, ask someone on the V8 team if it's something they can change the text of or if we should just rebaseline.

GURL difference - GURL tries to be compatible with how IE canonicalizes URLs. Sometimes, this differs from how KURL canonicalizes. If you're unsure about whether a test should be rebaselined, ask brettw.

Those tests that use NPAPI plugin require the plugin to be installed if you want to view the test from Chrome rather than the test_shell. To install a NPAPI plugin in Chrome, just copy the .dll into plugins directory next to your chrome.exe binary. If the plugins directory does not exist, you can create it.

Test Expectations

The webkit/tools/layout_tests/test_expectations.txt file contains the list of all known layout test failures. Unless it's a comment (C++ style), each line has the following format:

disrupts running of the other tests. Although this is not typical, it may still be possible. Please make sure to give Pri-1 to the associated bug.

Try to specify platforms and configs as accurately as possible. If a test passes on all but on platform, it should only have that platform listed

If a test fails intermittently, use multiple expectations.

Tracking Test Failures

All bugs, associated with layout test failures must have the LayoutTests label. Depending on how much you know about the bug, assign the status accordingly:

Unconfirmed -- you aren't sure if this is a simple rebaseline, possible duplicate of an existing bug, or a real failure

Available -- you know the root cause of the issue.

Assigned or Started -- you will fix this issue.

When creating a new layout test bug, please assign these labels to it -- having proper label hygiene is good for everyone:

Type-Bug

Pri-2 (Pri-1 if it's a crash)

Area-WebKit

OS-All (or whichever OS the failure is on)

LayoutTests

Mstone-4 (or current milestone)

FlakyTest (if the test is flaky)

Tests that use a HTTP Server

HTTP tests use a locally running lighttpd HTTP server to run. These tests dynamically construct an lighttp configuration file and start the server. You don't need any special configuration to run these, but you should make sure the server is not already running. As a rule, if the test is in an http directory, it will run off the server. If you want to run various tests manually while the local http server is running, note that it exposes different test suites on different ports. For example, all tests are available via port 8081, but the webkit tests are available via port 8000 (8443 for SSL) and the pending tests are available via port 9000 (9443 for SSL).

If you want to skip all these test, specify --nohttp on the command line. [FIXME: This option appears to have been removed... —snej 10/2009]

If you run the tests using run_webkit_tests.sh or .py, the server will be started automatically. To run the server manually:

$ cd src/webkit/tools/layout_tests

$ layout_package/http_server.sh --server start

To kill the lighttpd process, either run http_server.sh --server stop, or just use taskkill or the Task Manager on Windows, and killall or Activity Monitor on MacOS.

Debugging a test

After the layout tests run, you should get a summary of tests that pass or fail. If something fails unexpectedly (a new regression), you will get a test_shell window with a summary of the unexpected failures. Or you might have a failing test in mind to investigate. In any case, here are some steps and tips for finding the problem.

Take a look at the result. New tests come in with every WebKit merge, and sometimes we just need to rebaseline them (see below) to account for acceptable differences between Chromium and Safari.

Load the test into a trunk Chrome or test_shell build and look at its result. (For tests in the http/ directory, start the http server first. See above. Navigate to http://localhost:8000/ and proceed from there.) The best tests describe what they're looking for, but not all do, and sometimes things they're not explicitly testing are still broken. Compare it to Safari, Firefox, and IE if necessary to see if it's correct. If you're still not sure, find the person who knows the most about it and ask.

Some tests only work properly in test_shell, not Chrome, because they rely on extra APIs exposed there. So try that too.

Some tests only work properly when they're run in the layout-test framework, not when they're loaded into test_shell directly. The test should mention that in its visible text, but not all do. So try that too. See "Running the tests", above.

If you think the test is correct, confirm your suspicion by looking at the diffs between the expected result and the actual one.

Make sure that the diffs reported aren't important. Small differences in spacing or box sizes are often unimportant, especially around fonts and form controls. Differences in wording of JS error messages are also usually acceptable.

$ ./run_webkit_tests.sh LayoutTests/path/to/your/test.html --full-results-html will produce a page including links to the expected result, actual result, and diff.

Add the --sources option to run_webkit_tests.sh to see exactly which expected result it's comparing to (a file next to the test, something in platform/mac/, something in platform/chromium-win/, etc.)

If you're lucky, your test is one that runs properly when you navigate to it in test_shell normally. In that case, build the Debug test_shell project, fire it up in your favorite debugger (test_shell is single-process), and load the test file either from a file:// URL or by starting at the "WebKit Tests" link on the test_shell start page.

You'll probably be starting and stopping the test_shell a lot. In VS, to save navigating to the test every time, you can set the URL to your test (file: or http:) as the command argument in the Debugging section of the test_shell project Properties.

If your test contains a JS call, DOM manipulation, or other distinctive piece of code that you think is failing, search for that in the Chrome solution. That's a good place to put a starting breakpoint to start tracking down the issue.

Otherwise, you're running in a standard message loop just like in Chrome. If you have no other information, set a breakpoint on page load.

If your test only works in full layout-test mode, or if you find it simpler to debug without all the overhead of an interactive session, start the test_shell with the command-line flag --layout-tests, followed by the URL (file: or http:) to your test.

In VS, you can do this in the Debugging section of the test_shell project Properties.

Now you're running with exactly the same API, theme, and other setup that the layout tests use.

Again, if your test contains a JS call, DOM manipulation, or other distinctive piece of code that you think is failing, search for that in the Chrome solution. That's a good place to put a starting breakpoint to start tracking down the issue.

If you can't find any better place to set a breakpoint, start at the TestShell::RunFileTest() call in test_shell_main.cc, or at shell->LoadURL() within RunFileTest() in test_shell_win.cc.

Debug as usual. Once you've gotten this far, the failing layout test is just a (hopefully) reduced test case that exposes a problem.

Tips

Check the layout-test history page to see how a test did in the most recent ~100 builds on each builder (as long as the page is being updated regularly). (This page is only available from within Google for now, but it will be fixed up and made public as soon as possible. In the meantime, anyone with access will be happy to look up your test for you.)

A timeout will often also be a text mismatch, since the wrapper script kills the test_shell before it has a chance to finish. The exception is if the test finishes loading properly, but somehow hangs before it outputs the bit of text that tells the wrapper it's done.

Why might a test fail (or crash, or timeout) on the buildbot, but pass on your local machine?

If the test finishes locally but is slow, more than 10 seconds or so, that would be why it's called a timeout on the bot.

Otherwise, try running it as part of a set of tests; it's possible that a test one or two (or ten) before this one is corrupting something that makes this one fail.

If it consistently works locally, make sure your environment looks like the one on the bot (look at the top of the stdio for the webkit_tests step to see all the environment variables and so on).

If none of that helps, and you have access to the bot itself, you may have to log in there and see if you can reproduce the problem manually.

Rebaselining Tests

To automatically re-baseline tests across all Chromium platforms, using the buildbot results, see the Rebaselining Tool. Alternatively, to manually run and test and rebaseline it on your workstation, read on.

By default, text-only tests (ones that call layoutTestController.dumpAsText()) produce only text results. Other tests produce both new text results and new image results (the image baseline comprises two files, -expected.png and -expected.checksum). So you'll need either one or three -expected.* files in your new baseline, depending on whether you have a text-only test or not. If you enable the --no-pixel-tests, only new text results will be produced, even for tests that do image comparisons.

$ cd src/webkit/tools/layout_tests

$ ./run_webkit_tests.sh --new-baseline LayoutTests/foo/bar/test.html

The above command will generate a new baseline for LayoutTests/foo/bar/test.html and put the output file in the right place. This would be src/webkit/data/layout_tests/platform/chromium-{win,mac,linux}/LayoutTests/foo/bar/test-expected.{txt,png,checksum} for the above example.

When you rebaseline a test, make sure your commit description explains why the test is being re-baselined. If this is a special case (i.e., something we've decided to be different with upstream), please put a README file next to the new expected output explaining the difference.

Known Issues

Windows & Linux : Do not copy and paste while the layout tests are running, as it may interfere with the editing/pasteboard and other clipboard-related tests (Mac tests swizzle NSClipboard to avoid any conflicts).

If QuickTime is not installed, the plugin tests fast/dom/object-embed-plugin-scripting.html and plugins/embed-attributes-setting.html are expected to fail.

If Java is not installed, some tests will fail.

Mac: command line option --gdb is required to use gdb on TestShell. This prevents TestShell from trying hard to not hammer on Crash Reporter.